The present invention relates to an electric power steering apparatus using an electric motor as an auxiliary steering force generating means and, more particularly, to an improvement in an electric power steering apparatus capable of preventing generation of gear teeth striking noise or the like caused by a backlash upon meshing between gears in a gear mechanism which constitutes a rotational force transmission system for transmitting a rotational driving force of an electric motor to a steering shaft.
Various types of electric power steering apparatuses using an electric motor as an auxiliary steering force generating means have been proposed as described in, e.g., Japanese Patent Laid-Open No. 61-226362. Since an electric apparatus of this type has an arrangement simpler than those of conventional hydraulic apparatuses, a lightweight, compact arrangement of the entire apparatus can be realized. In addition, the apparatus can transmit a proper and reliable auxiliary steering force to vehicle wheels in accordance with a steering operation of a driver or various driving conditions of a vehicle. Furthermore, since the apparatus need be driven when a steering operation must be performed, energy saving can be achieved.
In order to adopt an electric motor as the auxiliary steering force generating means as described above, however, since various problems are present in, e.g., a structure of coupling the motor to a steering system and operation performance such as responsiveness including operation control of the motor, there is still large room left for improvements in the respective parts of the apparatus.
For example, one problem in an arrangement of the above electric power steering apparatus is a backlash generated between a pinion on a motor shaft of an electric motor and a gear wheel on the driven side which meshes with the pinion and is present on a steering wheel side. That is, in such a gear mechanism for transmitting a rotational force, a backlash is naturally necessary between gears in order to properly and reliably perform transmission of rotation. If, however, this backlash is unnecessarily large, a play is generated in a meshed portion between the gears to generate teeth striking noise or rattling noise when a steering operation is performed in a counterclockwise or clockwise direction. In addition, since a torque variation is easily generated due to meshing, poor responsiveness may be undesirably caused during transmission of rotation. This will be easily understood because the backlash between a pinion of a motor and a gear wheel on the driven side is increased also by a dimensional error in each part of a gear mechanism or a gap required to ensure movement of the part. On the other hand, a steering shaft of a steering wheel is coupled to an input shaft by a universal joint or the like with a predetermined angle therebetween. Therefore, the input shaft and an output shaft coupled to the input shaft via a torsion bar are swung due to bending or vibration of the steering shaft caused during a steering operation to unnecessarily increase the backlash. Therefore, generation of the play described above cannot be avoided.
When the backlash is increased to increase the play as described above, a motor is driven in a forward or reverse direction accordingly to generate an idling state during transmission of rotation. Therefore, if a vehicle runs on a rough road or runs onto curbstones during driving, vehicle wheels are turned by a force of action from the ground surface, and an impact rotational force so-called "kick back" is applied as a reverse input from the vehicle wheels. As a result, problems in which, e.g., gears strike each other to generate teeth striking noise or the like, are posed.
If the backlash is zero, it is naturally possible to avoid generation of a play and solve the problem of teeth striking noise. Since, however, not only high machining accuracy is required to obtain a zero backlash, but also transmission of rotation performed by meshing between the gears becomes impossible. Therefore, the zero backlash is practically impossible.
The present applicant, therefore, has previously proposed an arrangement by, e.g., U.S. Ser. No. 460,779 (abandoned) in which a gear wheel such as a hypoid gear having a gear portion on its one side surface is axially supported on a steering shaft so as to be slidable along the axial direction, and biasing means such as a coil spring, a leaf spring, or a wave washer is provided to bias the gear wheel in a direction to mesh the gear wheel with a pinion. In this arrangement, a backlash in a portion of the gear wheel to be meshed with the pinion can be adjusted to be almost zero in a non-steering state.
In this conventional structure, however, the entire gear wheel is merely biased on the shaft toward the pinion. Therefore, in a hypoid gear as a gear pair with non-parallel, non-intersecting axes in which a gear wheel and a pinion are arranged such that the pinion meshes with some teeth of a gear portion formed on the side surface of the gear wheel, the gear wheel is shifted with respect to the shaft due to a difference between forces of action in the axial direction of portions to be and not to be meshed with the pinion. As a result, a so-called falling phenomenon may be caused in a gap formed in accordance with machining accuracy or the like in an axially supported portion of the gear wheel with respect to the shaft. If such a falling phenomenon occurs, occurrence of a pinching or catching phenomenon between the axially supported portion of the gear wheel and the shaft cannot be avoided to pose problems such as a play in the gear wheel, generation of teeth striking noise caused by the play, a torque variation, and locking between the teeth of the gear wheel and the pinion. Therefore, a demand has arisen for a countermeasure capable of solving these problems.